1. Technical Field of the Invention
The present invention relates to an apparatus having a plurality of computers, particularly personal computers (PC's), for the selective transfer of data from such computers to, at least, one shared output device, e.g., a printer, or for the file transfer between the computers by way of a parallel interface.
2. Description of the Prior Art
Known to the prior art, there are devices for the connection of one or more shared printers to a plurality of PC's, whereby the parallel interface of each PC is connected over a conventional parallel printer cable to a printer sharing device (PSD), which in turn is connected over a conventional parallel printer cable to the parallel interface of the printer. Each PC can now transmit data to the printer. If the PSD possesses a memory, the PC's may also transmit data simultaneously. While a PSD is required, the shared printer affords considerable cost savings when compared to PC's equipped with their own printer. In order to transfer files between the PC's over the parallel interface, it is necessary to disconnect the parallel printer cables and connect the interfaces with the special cable included with the file transfer program. Hence, the switch-over from data transfer to file transfer between the computers entails a re-connection of the cables, or else a second parallel interface must be installed in the PC's, thereby permitting simultaneous connection of the conventional printer cable for the transfer of data to the printer and the special cable for the transfer of files to another computer. In practice, this is a time-consuming and most undesirable practice.
Originally, the parallel interface was designed solely for unidirectional data transfer from the computer to the printer. (Bi-directional variants of the parallel interface have since been developed, but they lack standardization and are by no means available for all computers.) Hence, computers have only one data output and printers have only one data input. In addition, there are some control circuits to the printer and some status lines between the printer and the computer, monitoring the status of the printer. Therefore, any file transfer program wishing to transfer data to the computer over a parallel interface would have to utilize the status line from the printer as a data communication line to the computer. To begin with, this would require a special cable with a cross-over conductor allocation; secondly, there is no standard for it. Any supplier of such a program could select his own cable allocation so that, in theory, there could be a great many different allocations.